Electrographic printer with plural oscillating print head

ABSTRACT

A high speed, electrographic, non-impact printer for printing matrix-type characters on the dielectric coated face of a paper web. A print head having a plurality of linearly arranged styli therein is transported in an oscillatory fashion across the dielectric coated face of the web with the styli in virtual contact therewith. Means are provided for selectively energizing the styli for depositing dots of electric charge in image configuration on the dielectric as the print head traverses the web in both directions. The position of the print head as it traverses the web in both directions is sensed for controlling the selective energizing means for depositing dots of electric charge on the dielectric only when the print head is in predetermined positions in its path across the web. The dots of electrical charge are developed by means of a toner system.

ELECTROGRAIIIIC PRINTER WITH PLURAL OSCILLATING PRINT HEADS Filed:

Inventors: Robert H. Detig, Tappan, N..1.;

Bertrand Boyson, Manhasset, NY.

Assignee: Ing. C. Olivetti & C0., S.p.A.,

Ivrea, Italy Feb. 18, 1971 4 Appl. No.: 116,361

[52] US. Cl. 197/1 R, lOl/DIG. 13, 346/74 ES [51] Int. Cl.G0ld 15/06,GOld 15/08, GOld 15/012 [58] Field of Search l0l/DIG. l3; 197/1; 340/74ES [56] References Cited UNlTED STATES PATENTS 2,881,976 4/1959 Greanias1 235/61 3.042.174 7/1962 Howard r 197/49 3,045,587 7/1962 Schwertz.....lOl/DIG. 13 3,167,166 1/1965 Schiebeler 197/1 R 3,289,209 11/1966Schwertz et a1. lOl/DIG. 13 3,292,530 12/1966 Martin 101/93 C 3,420,166l/l969 Ellis et a1. 101/93 C 3,426,880 2/1969 Blodgett 197/1 R Bresler197/1 R Finnegan 197/1 R Primary Examiner-Edgar S. Burr Attorney-KevinMcMahon [57] ABSTRACT A high speed, electrographic, non-impact printerfor printing matrix-type characters on the dielectric coated face of apaper web. A print head having a plurality of linearly arranged stylitherein is transported in an oscillatory fashion across the dielectriccoated face of the web with the styli in virtual contact therewith.Means are provided for selectively energizing the styli for depositingdots of electric charge in image configuration on the dielectric as theprint head traverses the web in both directions. The position of theprint head as it traverses the web in both directions is sensed forcontrolling the selective energizing means for depositing dots ofelectric charge on the dielectric only when the print head is inpredetermined positions in its path across the web. The dots ofelectrical charge are developed by means of a toner system.

12 Claims, 18 Drawing Figures Patented Aug. 14, 1973 8 Sheets-Sheet 3Patented Aug. 14, 11973 8 Sheets-Sheet 4 Patented Aug. 14, 1973 8Sheets-Sheet 6 ATTO R N EY Patented Aug. 14, 1973 8 Sheets-Sheet 7 -iILATTO R N EY ELECTROGRAPHIC PRINTER WITH PLURAL OSCILLATING PRINT HEADSBACKGROUND OF THE INVENTION This invention relates generally tohigh-speed electrographic non-impact printers and more particularly tohigh-speed electrographic non-impact printers having oscillatingprinting heads.

Impacting mechanical high-speed printers have long been used forproducing computer printout. These printers are capable of printing anentire line of type essentially in parallel and the more expensive onesare adequately fast for most present day applications.

They do, however, have several serious drawbacks. They are extremelynoisy machines and must therefore be isolated, usually in a separateroom, from people who are expected to work. They are also very complexand extremely expensive and are subject to a fairly high rate ofmechanical breakdown.

Recently electrographic line printers have been introduced which solvesome of the problems. These printers deposit electrostatic charge latentimages of the information to be printed on a web of dielectric coatedpaper. The electrostatic image is developed by means of liquid or drytoner developing systems. These machines usually include a stationaryprinting head assembly having as many as 1,000 or more individual styliarranged in contact with the coated side of the web in a line normal tothe direction of its movement. A common electrode is also provided,usually on the opposite side of the web and, as the web is stepped pastthe styli, dots of electrostatic charge are deposited on the dielectriccoating by selectively pulsing the styli. Printers of this type areusually arranged to print a character by depositing charge on selecteddots of a X 7 dot matrix.

Line printers of this type offer a significant step forward over theearlier impacting kind. They are very quiet in operation, print at ahigh rate of speed, and produce quite acceptable print quality.

There are, however, many problems with these machines that still exist.Since they require an individual high voltage switch for each of the1,000 or more styli, and also the electronics for controlling theactivation of all the styli in parallel, these machines are extremelyexpensive and quite complex electronically. This electronic complexityalso severely limits the reliability of the machine.

A further problem with these known printers is that there is a tendencyto print doughnuts," (i.e., dots having only the periphery black) unlessa rather expensive grade of paper is used which has a very smooth anduniform coating. This is thought to be because the styli tend to depositcharge on the paper only around this periphery unless the paper surfaceis quite smooth. Dots of this type appear light and faint to the eyesince the eye tends to integrate the'black circle with the whiteinterior.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of theinvention to improve electrographic line printers.

It is a further object of the invention to increase the reliability ofand to simplify electrographic line printers by reducing the number ofstyli in the printer.

It is another object of the present invention to provide anelectrographic printer which prints solid, dark appearing dots.

In accordance with these and other objects of the invention there isprovided a high speed electrographic line printer for printing lines ofinformation on a web having a charge retaining surface normal to thedirection of its movement which includes a plurality of print headsmounted for movement across the web in contact therewith, and means formoving the print heads in an oscillatory fashion across said paper. Alsoprovided are means mechanically coupled to the print heads for movementtherewith for sensing the position thereof and means for selectivelyactivating the styli for depositing a line of character-shapedelectrostatic latent charge images on said paper for each sweep of saidstyli across said paper in either direction, and means for developingsaid latent images on said paper with a visible toner.

Various other objects, advantages and features of the invention willbecome more fully apparent in the following specification with itsappended claims and accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of an embodimentof an electrographic printer according to the invention;

FIG. 2 is a back view of the printer of FIG. 1;

FIG. 3 is a schematic showing the mechanical drive for the various partsof the printer of FIG. 1;

FIG. 4 is a cut-away view of the print head and slider assembly;

FIG. 5 is a detail drawing of the print head of FIG. 4;

FIG. 6 shows the printing face of the print head of FIG. 5;

FIG. 7 shows a spring-mounted print head according to the invention;

FIG. 8 is another view of the spring-mounted print head of FIG. 7;

FIG. 9 shows a second embodiment of a springmounted print head accordingto the invention;

FIG. 10 is another view of the spring-mounted print head of FIG. 9;

FIG. 11 showsan embodiment of the invention in which the print head isintegral with the slider;

FIG. 12 shows the position sensing device according to the invention;

FIG. 13 is a sectional view of the vice of FIG. 12;

FIG. 14 is a schematic diagram of the electronic drive and conrolcircuitry for the printer according to the invention;

FIG. 15 is a more detailed schematic of a portion of the controlcircuitry of FIG. 14;

FIG. 16 serves the control circuitry for the motors and power suppliesof the printer according to the invention;

FIG. 17 shows an embodiment of the device for sensing when the slider isat the end of the line;

FIG. 18 is a schematic diagram of a high voltage amplifier for driving astylus of a print head according to the invention.

DETAILED DESCRIPTION The invention can best be understood by referringto the following detailed description of the illustrated embodiments.

position sensing 'de- Referring to FIG. 1 of the drawings, there isshown an embodiment of an electrographic line printer according to theinvention. The printing is performed on dielectric coated paper 11 whichis threaded over the roller 13 from the back of the machine throughsuccessive stations where it is imaged, developed, dried and finallydisplayed. The latent electrostatic image is deposited on the dielectriccoating by means of four print heads 17 which are mounted for movementnormal to the plane of the drawing in a slider 19. The slider 19 slidesin ways 20. The grounding bar 21 operates both to hold the paper web 11firmly in contact with the vprint heads 17 and to act as the voltagereference for the activation of the styli of print heads 17. In order toinsure good contact between the print heads 17 and the web 1 1, thegrounding bar 21 should have a resilient or a solid surface facing theprint heads 17 depending on whether the print heads 17 are fixedly orresiliently mounted respectively.

After being imaged, the paper web 11, which, for this embodiment of theinvention has perforated feed holes along either side thereof, is fed bymeans of tractors 23 mounted for engagement with the perforated feedholes on the web 11 to the developing station 25.

The dielectric coated paper used in the printer according to theinvention may be of the standard type used for electrographic printingand is available from several manufacturers. Preferably the coatingshould be such that it does not flake or smear and thereby interferewith the operation of the print heads 17.

In the illustrated embodiment of the invention, the latent image on thepaper web 11 is developed by means of a liquid developer system' whichbrings the developer into contact with only the imaged side of the web11 by means of the developing roller 27. The developer roller 27 isrotated at a high speed (on the order of 850 to 1,500 rpm) in an innertray 29 which is partially full with the liquid developer. The roller 27picks up a wave of this developer and brings it into contact with thelatent image on the dielectric coated side of the web 1 1. The web 11 isheld slightly out of contact with the roller 27 by means of the roller31. The spacing between the web and the roller 27 may be on the order ofbetween 0.075 and 0.125 inches.

In order to keep the toner concentration in the developer uniform and atthe proper level, the developer may be constantly recirculated.An'overflow port (not shown) may be provided in the inner tray 29 toallow the developer to overflow into an outer tray 33. A drain tube 35is provided for the outer tray 33 to allow the developer to flow at apreselected rate from the outer tray to a developer reservoir 37.Developer is pumped from the reservoir 37 back to the inner tray 29 bymeans of pump 39.

The volume of and toner concentration in the developer in tank 37 dropsduring use at differentrates and must be controlled. In order to holdthe volume of developer in tank 37 constant, developer at the properconcentration is metered into the developer tank 37 from the container38 by means of a pump 40. The actuation of the pump 40 is controlled bya switch (not shown) which senses the level of the developer in the tank37 and activates the pump 40 when the developer level falls below apredetermined level. The concentration of the developer is sensed bywell known means (not shown) and concentrated developer is metered intotank 37' from a concentrate container (not shown) as needed.

The use of the wave of developer on the roller 27 to develop the imageon the web 11 has the advantage that, when the system is not in use, nodeveloper is in contact with the paper. This avoids the presence ofunsightly black splotches across the paper.

It should be apparent that other developing systems may also be used todevelop the latent image on the paper web 11. Liquid fountain developingsystems or magnetic brush dry toner systems, for example, would each beapplicable.

In transporting the web 11 through the developing system 25, the rollers41 and 43 receive the web 11 after it leaves the tractors 23. Guides 42and 44 act to guide the leading edge of the web 11 around the lower sideof roller 31.

After being developed by the roller 27, the web 11 passes throughsqueegee rollers 45 and 47 in order to remove the excess developertherefrom. The squeegee rollers 45 and 47 may be of standard type. Inthe preferred embodiment of the invention, roller 45 is metal, whileroller 47 is a rubber roller. It is possible touse a rubber roller forroller 45 also. However, under some conditions, some of the image may betransferred from the paper onto the rubber roller which will thenreprint on a following portion of the web 11 on the next revolution ofthe roller 45. The use of a metal roller avoids this problem. The excessdeveloper which is removed from the web 11 by the squeegee rollers 45and 47 drops into the outer tray 33.

After leaving the squeegee rollers, the web passes through guides 49 and51 which guide the leading edge of the web 11 to the nip between rollers53 and 55. Roller 53 also serves as one of the guide rollers for an 0"ring conveyor 57. The 0" ring conveyor passes over guide rollers 53, 59and 61 and serves to transport the web past a drying blower 63 to aviewing station 65. After the web 11 is viewed it is carried rearwardlyto the back of the machine. 7

Rollers 67 and 69 act in conjunction with rollers 49 and 61respectively, for guiding the web on the 0 ring conveyor 57. I

In the illustrated embodiment of the invention, the drying of the web isperformed by the blower 63, which directs a stream of hot air at the web11. The heating of the air may be performed by an electric heatingelement (not shown) disposed in the air stream of the blower. It wouldbe equally possible to perform this drying function by the use of aheated roller, which the paper is wrapped around. Roller 59, forinstance, could easily perform this function if a heating element weredisposed therein.

Referring now to FIG. 2 of the drawings, there is illustrated a rearview of the electrographic line printer according to the invention,showing the driving mechanism for the slider 1.9. In the illustratedembodiment of the invention, the slider 19 carries four spaced printingheads 17, each of which images one-fourth of the width of the paper web11. It is, of course, possible to use a different number of printingheads 17 in other embodiments of the invention. In practice there is atrade off between the amplitude of the oscillation of the slider 19 andthe complexity of the electronics necessary to control the printing. Thefewer printing heads 17, the simpler the electronics, but the larger theamplitude of oscillation of the slider 19 and, therefore, the lower thespeed.

In the illustrated embodiment, the slider 19 is driven in anapproximately sinusoidal manner by means of the cable 71 and the disccrank 73. The cable 71 extends from one end of the slider 19 around thepulley 75 to the pin 77 on the disc crank 73. From pin 77, the cable 71extends around pulley 79 to the other end of the slider 19. The spring81 is provided to allow for the difference in length of the cable 71 foreach 90 rotation of the disc crank 73.

During operation of the printer, the disc crank 73 is driven at aconstant rotational speed through a clutch/- brake mechanism 83 (FIG. 1)by means of a motor. If the printer is operating at a printing speed of600 lines per minute, the disc crank 73 must rotate at 300 cycles perminute.

The illustrated type of slider drive was chosen because of itssimplicity and low moving mass. Other drives are of course alsopossible, however. A crank drive, or a pin-chain type drive, forexample, could also be used. Some of these drives may drive the sliderin a linear fashion as opposed to the sinusoidal fashion as in thepresent embodiment.

The mechanical drive for the various portions of the high speed printerillustrated in FIGS. 1 and 2 of the drawings is shown in FIG. 3. Papertractors 23 are driven through gears by stepper motor .83 for steppingthe web 11 past the print heads 17 in slider 19. The pause between eachstep must be long enough for the print heads 17 to traverse the web 11once and the stepping speed must be fast enough to advance the web byone line in the turn-around time of the slider 19. In the illustratedembodiment the web 11 may be advanced at a speed on the order of 600lines per minute or more.

As explained above, the slider 19 is driven by means of disc 73 throughthe cable 71 (FIG. 2). The disc 73 is driven through a magneticclutch/brake mechanism 83 by a drive motor 87. During printing the drivemotor 87 is operated at a constant speed so that the slider 19 traversesthe web 11 at the desired frequency. For instance, if the printer is tooperate at a 600 line per minute rate, the slider 19 must operate at 300cycles per minute.

As will be explained in more detail later, means are provided forsynchronizing the operation of stepper motor 85 and the slider 19 sothat the web 11 remains still while the slider 19 traverses it. Themagnetic clutch/brake mechanism 83 is provided so that, if it is desiredto leave portions of the web 11 blank, the slider can be held immobilewhile the stepper motor 85 moves the web 11 at a faster rate.

The drive for the transport rollers of the processor portion 25 of theprinter is provided by the processor motor 89 through a slip clutch 91.The slip clutch 91 is coupled by means of belt 93 to the transportroller 55 and to belt 95. Belt 95 in turn drives roller 45 and belt 97.Belt 97 is used to drive roller 43. the processor motor 89 operates at aspeed which slightly overdrives the rollers of the processor 25 in orderto keep the web 1 1 tensioned over roller 31. The slip clutch 91prevents the rollers of the processor 25 from damaging the web 11 byinsuring that they do not exert too much tensioning force on it.

The toning roller 27 is driven by the motor 99. The roller 55 of the 0"ring conveyor drives roller 53 through gears 58 by means of the motor89.

The structure of the slider 19 and print head 17 is illustrated in moredetail in FIG. 4 of the drawings. The slider 19 is of trapezoidal crosssection and slides in the ways 20 on either side of the slider. Each ofthe ways 20 consists of a lower portion having a part out out to allowthe passage of the print heads 17 and a flat upper surface on which thebase of the slider 19 rests. Each of the ways 20 also has an upperportion 107 attached to the lower portion 105 by means of bolts 109. Theupper portion 107 includes an angled surface 111 which bears against theangled face of the slider 19 and holds it against both vertical andhorizontal movements. This slider construction has the advantage ofallowing the adjustment of both the horizontal and vertical forces onthe slider 19 by the adjustment of only the bolts 109.

Various structures may be used for the print head 17 (FIG. 1) andseveral embodiments are illustrated in FIG. 5 through FIG. 11 in thedrawings. An embodiment ofa print head 17 which may be made by printedcircuit techniques is illustrated in FIG. 5 of the drawings. Accordingto this embodiment, the conductors 111 which form the seven styli of theprinting head are etched on a printed circuit backing board 113 whichmay be formed of a polyphenylene oxide base material marketed under thetrade name Z-tron by Polypenco Inc., or on a polyester such as Mylar(trademark). The conductors 111 which form the seven styli run from theprinting face 115 of the printing head 17 back to the interconnectionportion 117 of the printing head 17. Here a series of seven individualinterconnection pads 119 are formed for connecting the individual styli111 to the drive circuitry.

As can be better seen by FIG. 6 in conjunction with FIG. 5, the endportion 121 of writing head 17 consists of a laminate of several layers.The styli 111 are covered with a covering layer 123 which may be made ofthe same material as the substrate 113. The covering layer 123 may beglued to the styli 111 and substrate 113 by an epoxy-type glue, whichglue also fills in the interstices between the adjacent styli 111. Metalplates 125, which may be formed of stainless steel, ceramic orglass-filled plastic, are attached to either side of the end portion 121of the writing head 17. These plates 125 act to increase the mechanicalstrength and stability of the end portion 121, and also make for anextremely low wear, low friction printing face 1 15. The plates 125 maybe attached to the end portion 121 by means of an epoxy glue or bybolts.

After the end portion 121 is laminated, the printing face 115 is groundin order to provide a very smooth flush face. Preferably the edges ofthe plate 125 are beveled in order to allow the printing head 17 to camover the edge of the paper web 11 when the printing is being performedon a narrow web 11.

One problem encountered with the printer of the invention is thetendency of the writing head 17 to clog with particles from thedielectric coating on the paper web 11 if the stylus design is notcorrect. These particles are thought to result from the sliding contactbetween the printing heads 17 and the paper. The stylus designsdescribed in relation to FIGS. 5 and 6 has been found to minimize thisflaking tendency and thereby give good results.

The printing head 17 may be fixedly mounted in the sliders 19 asindicated schematically in FIG. 1. In this case it has been foundpreferable to have the printing head 17 extending slightly (0.005 inchhas been found suitable) from the slider 19. This is preferable both todecrease the frictional contact between the slider and print headassembly and also because, if they were mounted flush with the slider19, any particles from the coating of the web 11 getting between theslider 19 and the web 11 might bring the printing heat 17 out of contactwith the paper and thereby cause a malfunction.

Although, as stated above, the writing head 17 may be fixedly mounted inthe slider 19, it has been found preferable to spring-mount them asshown in FIGS. 7 and 8 of the drawings. According to this embodiment,the end portion 121 of the writing head 17 is slidably mounted in aclose fitting aperture in the slider 19. A spring 127 is connectedbetween an extension 129 attached to the printing head 17 and a mountingpiece 131 attached to the slider 19 for forwardly biasing the writinghead 17.

The use of the spring-mounted writing head 17 has been found to improveprinting quality. It is thought that this is because it insures goodcontact between the writing face 115 and the web 111. The strength ofthe spring 127 is chosen so that, during printing, the printing face 115extends about 0.005 inch from the surface of the slider 19.

Another embodiment of a spring-mounted writing head is illustrated inFIGS. 9 and 10 of the drawings. According to this embodiment, thewriting head 133 is formed of a slug of plastic material having arectangular cross section which is slidably mounted in a close fittingaperture in the slider 19. A mounting member 135 which may be bolted tothe slider 19 holds one end of a compressed spring 137, and the otherend of the spring 137 bears against a plate 139 which may be formed aspart of the writing head 133. The spring 137 biases the writing head 133in the forward direction so that it extends slightly from the surface ofthe slider 19. According to this embodiment the writing head 133 and thebearing plate 139 may be formed as a single piece by injection moldingtechnique out of a suitable plastic material. In order to be suitable,the plastic material must wear very slowly when in contact with thepaper and have a veryv low coefficient of friction. One material whichhas been found to be suitable is Delrin AF, which is a teflon-filledDelrin marketed by DuPont. Holes for the styli 141 may be drilled in thehead. The styli are then inserted in the holes and potted. An extension143 of the printing head 133 may be provided to carry the styli 141 backthrough an aperture 145 in the mounting bracket 135. Connections aremade to the styli at the back face of the extension 143. Good resultshave been achieved with printing heads constructed according to theembodiment illustrated in FIGS. 9 and 10. This embodiment has theadvantage of being somewhat simpler and cheaper to make than thatdescribed in relation to FIGS. through 8.

It is also possible to form a printing head such as shown in FIGS. 9 andin two halves split at the line A A in FIG. 10. In making the printinghead according to this method seven parallel axial channels forreceiving the seven styli 141 are formed in one-half of the writing headin the face A A which is then coated with copper of such a thicknessthat the channels are filled in. The face A A is then ground down toremove the copper plated on the face, thereby leaving only the sevenindividual styli in the channels. The two halves of the writing head maythen be attached together, for instance by means of an epoxy glue.

In FIG. 11 there is shown still another embodiment in which the need fora separate writing head is eliminated altogether. According to thisembodiment, the slider 19 is itself the writing head. For each of thefour writing heads a line of seven holes is drilled in the slider 19, asis illustrated in FIG. 11. The styli 147 are then inserted into theholes and fixedly mounted therein, preferably by means of an epoxy typeglue. The face 149 of the slider 19 is then ground so that it provides avery flat, smooth surface with the tips of the styli being flushtherewith.

In this embodiment the material of the slider 19 must have a very lowcoefficient of friction so that it does not cause the generation ofparticles from the coating on the web 11. Delrin AF mentioned above hasbeen found to be a suitable material for the slider 19 in thisembodiment.

With this embodiment of the writing head and slider assembly, it ispreferable that the grounding bar 21 (FIG. 1) have a flexible resilientsurface facing toward the web 11 in order to insure good contact betweenthe styli 147 and the web 11.

An eighth stylus 150 may also be included in the printing head of FIG.11, or for that matter in any of the printing head configurationsillustrated in FIGS. 4 through 10. This eighth stylus 150 may be usedfor underlining selected portions of the information being printed.

Print heads similar to that illustrated in FIGS. 9 and 10 of thedrawings may be constructed using a flat wire cable which is assembledwith supporting metal or plastic side pieces. The printing face of theprinting head should be ground so that the conductors are flush with theface.

An important feature of the printer according to the invention is thatthe dots of the characters are printed while the print heads 17 aremoving across the web 11 rather than while stopped. As a result of this,the problem of the tendency of prior art printers to print doughnuts issolved since the edges of the styli are wiped across the web 1 1 duringthe time they are energized for depositing charge and thus deposit dotshaving a uniform charge distribution which, when developed, appear darkand solid.

In order to maximize the uniformity of the charge distribution, it ispreferable to use styli with a thin rectangular cross section as opposedto ones with a round cross section.

Since, during printing, the motion of the slider 19 is approximatelysinusoidal, it is not possible to time the enabling of the print headsfor printing the dots of the successive columns with afree-running-clock signal. Rather, an optical position encoderillustrated in FIGS. 12 and 13 of the drawings, is used in conjunctionwith the slider 19 for generating timing pulses to enable the printingof the successive columns of dots by the print head 17.

In the illustrated embodiment of the invention each print head 17 prints32 characters. Each character position contains seven columns positions,five columns positions of which are used to form the 5 X 7-dot charactermatrix and the other two serving as the intercharacter spaces. It isnecessary to generate 224 position signals each time the slider 19traverses the web 11. FIG. 13, which is a sectional view of FIG. 12,along the line A A shows the circuitry which may be used to sense theposition of the slider 19. Light from the bulb 151 is collected by abundle of glass fibers 153 and passed through a slit 155 in housing 157.The slit 155 may, for instance, be 150 microns wide by 0.5 inch high.Light from the slit 155 is interrupted by a transparent mask 159, whichis fixedly attached to the slider 19. The mask 159 has 224 parallelblack lines 161 ruled on it, each of which may be 175 microns wide by0.5 inch high. The lines may be spaced from each other by 175 microns.

The light passing through the mask 159 passes through a slit in housing163 which may be identical to the slit 155 in housing 157, and whichmust be directly opposite slit 155. The light is then collected by afiber optic bundle 165 which may be substantially identical to thebundle 153, and fed to the input of a phototransistor 167. This outputis amplified by amplifier 169. The output amplifier 169 may then be usedto enable the printing of the successive columns of dots by the printingheads 17.

Each time the light source 151 is interrupted by one of the lines 161 avoltage pulse appears at the output of the amplifier 169. This pulse maythen be used to enable the printing heads 17 for printing the successivecolumns of dots.

The phototransistor 167 may, for instance, be a Fairchild type FPT-IOO.

If the drive used for the slider imparts a uniform speed to it,theposition encoder would not be necessary.

An embodiment of circuitry which may be used to control the printeraccording to the illustrated embodiment of the invention is illustratedin FIG. 14 of the drawings. The data to be printed is received in aserialby-bit fashion from a computer and fed into the six-bitserial-to-parallel converter 171. From the converter 171 the data istransferred one six-bit character at a time to the one-line buffer 173.The one-line buffer 173 holds a full line of 128 six-bit characters tobe printed on a single sweep of the print head 17 across the web 11. Thesix-bit code for the successive characters to be printed by the printheads 17 are sent by the buffer 173 to the individual character decoders175. Four of these character decoders are provided, one for each of theprint heads 17.

The character decoders 175 also receive five inputs from the seven-bitshift register 177. Shift register 177 is controlled by the sliderposition sensor whose operation is described in relation to FIGS. 12 and13. The pulse output of the amplifier 169 is first shaped by the squarewave shaper circuit 179 and then used to control a monostablemultivibrator 181. Monostable multivibrator 181 in turn controls theseven-bit shift register 177.

The seven-bit shift register 177 therefore shifts each time the printheads 17 come into position for printing the next column of dots. Theoutputs 2 through 6 of the shift register 177 correspond to the fivecolumns of the X 7 character matrix. The outputs 1 and 7 correspond tothe intercharacter spaces.

The outputs 2 through 6 of the shift register 177 are fed to thecharacter'decoders 175. The decoders 175, on the basis of the six-bitcharacter code from the buffer 173 and the activated column input fromthe shift register 177, generate drive pulses to selected ones of thecorresponding set of seven high-voltage amplifiers 183. The outputs ofthe seven high-voltage amplifiers are used directly to activate theseven styli of the print heads 17.

The web advance stepper motor (FIGS. 1 and 14) is controlled through thestepper driver circuitry 185. The web 11 is line-spaced by the steppermotor 85 during the time that the stylus and print head assembly ischanging direction at the end of a sweep across the web 11. When aportion of the web 11 is to be left blank, the slider 19 is stopped atone end of its sweep by the clutch/brake mechanism 83 (FIGS. 1 and 2)and the driver circuitry 185 pulses the stepper motor 85 at a high rateso that the web 11 is moved quickly past the slider 19 for that portionwhich is to be left blank.

The control and timing signals for the stepper motor driving circuitry185, the clutch/brake 83, and the character data handling circuitry areprovided by the control unit 187. The control unit 187 receives controlsignals from the computer to which the printer is coupled and in turncontrols the flow of data received by the printer from theserial-to-parallel converter 171 to the printing head driver circuitry183. The control unit 187 also controls the motion of the slider 19through the clutch/brake 83 and the movement of the web 11 through thestepper motor 85 and the associated drive circuitry 185.

The slider and printing head assembly sweeps across the web 11alternately from right-to-left and left-toright. It is thereforenecessary for the control unit 187 to control the data handlingcircuitry in such a manner that the data signals de-livered to the highvoltage amplifiers 183 cause the print heads 17 to print the successivelines of characters on the web 11 alternately forwards and backwards. Inthe illustrated embodiment of the invention, the data input from thecomputer to the converter 171 is in the left-to-right reading mode.

Therefore the control unit 187 must invert alternate lines of datasignals supplied to the high voltage amplifiers 183. This isaccomplished by inverting the order of character readout from the buffer173 to the character decoders and by inverting the order of energizationof the output by the seven-bit shift register 177, thereby inverting theorder in which the columns of dots for each character are printed by theprint heads 17.

A more detailed diagram of portions of the control unit 187 and thebuffer 173 is illustrated in FIG. 15 of the drawings. The buffer 173 mayinclude four sets 189 of six 152-bit end around shift registers 191.Each set 191 is associated with one of the character decoders 175 andprint heads 19. The bits of the six-bit character code for eachcharacter to be printed are stored in the corresponding bit location inthe shift registers 191. Thus the bits of the six-bit code for aparticular character to be printed may be stored, for instance, in thefirst, or third, or 32-bit location of each of the six shift registers191 of a set 189. The following character to be printed would be storedin an adjacent bit location of each of the shift registers 191.

When the printer is turned on by closing switch 193 (FIG. 16), the highvoltage power supply 281, the low voltage power supply 283, the sliderdrive motor 87, the developer roller motor 99, the processor motor 89,the developer pump 39 and the blower 63, shown in FIG. 16, are actuated.After a predetermined delay to allow these components to get over anytransient conditions the control logic 195 sends a ready signal to thecomputer on line 197 which tells the computer that the computer is readyto print and essentially turns over the control of the operation of theprinter to the computer.

The computer continually sends clock pulses on line 199 to the controllogic 195 for controlling the loading of the characters to be printedinto the buffer 173. While the computer is sending data to the printer,it also generates a voltage level on line 201 to the control logic 195.While this voltage level is present on line 201, the control logic 195transmits the clock signals received on line 199 to the six-bitserial-to-parallel converter 171 for enabling the serial-to-parallelconverter 171 to receive and store the contemporaneously received databits from the computer. The frequency of the enabling pulses being sentto the serial-to-parallel converter 171 is divided by six by thefrequency divider circuit 203 whose output is transmitted through ORgate 205 to shifting line 207, which causes the contents of the shiftregister 191 to shift one place to the right. This causes the contentsof the serial-to-parallel converter 171 to be transmitted through thegating circuit 209 to the first bit place of the six registers 191 of aset of registers 189 selected by the gating circuit 209.

Thus, each time the serial-to-parallel converter 17] is filled with asix-bit character code, its contents are transmitted through gatingcircuit 209 to the first bit place of a set 89 of shift registers 191.

The gating circuit 209 is controlled by the output of the frequencydivider circuit 211, which divides the frequencyof the shift pulsesreceived from the frequency divider 203 through OR gate 205 on line 207by 32. In this way the first 32 six-bit character codes received fromthe computer are successively loaded into the shift registers 191 of theset 189 which controls the leftmost print head 19. Therefore, when the32nd character is loaded into the first set 189 of shift registers 191,the six-bit code of the character to be printed in the leftmostcharacter position on the web 11 on the next sweep of the print head andslider assembly across the web 11 is located in the 32nd bit place ofthe registers 191 of the first set 189.

When the divider 211 signals the gating circuit 209 that the 32ndcharacter code is loaded into the first set 189 of shift registers 191,the gating circuit 209 switches so that the following characters areloaded in an identical manner into the second set 189 of shift registers191 which may be identical with the first set 189. The second set 189 ofshift registers 191 hold the information to be printed by the secondwriting head 19.

This process continues with the divider 21 l signalling the gatingcircuit 209 on every 32nd six-bit character code received so that thegating circuit 209 loads successive groups of 32 characters into thesuccessive sets 189 of shift registers 191 until all four sets 189 arefilled with 32 characters each.

When the computer has sent the 128 character field to the printer, thevoltage level on line 201 to the control logic 195 drops, therebycausing the control logic 195 to cease send-ing the enabling signals tothe serialto-parallel converter 171.

If the slider 19 is beginning its sweep across the web 11 from theleft-hand side of the web 11, the multivibrator 213 has its output whichis connected to AND gate set, thus enabling the AND gate 215 to transmitpulses generated on line 7 of the seven-bit shift register 177 (FIG. 14)to the OR gate 205 to act as shift pulses on the line 207 to the shiftregisters 191.

The output from the shift registers 191 is taken from the 32nd bit placeof each of the registers which, at the end of the loading operationdescribed above, stores the bits of the characters to be printed in theleftmost printing position of each of the print heads 19. Thus, thesix-bit codes of the first, thirty-third, sixty-fifth, andninety-seventh characters to be printed on the 128- character line arepresent on the input line of the character decoders from the buffer 173.

The operation of the printer electronics will first be described inrelation to the situation when the slider 19 is traversing the web 11from left to right. When the print heads 19 reach the position wherethey are to begin printing, the first line ruled on the transparent mask159 passes between the lamp 151 and the photodiode 167 causing a pulseoutput from the photo transistor 167. This pulse is amplified byamplifier 169 shaped by square wave shaper 179 and then used to triggerthe monostable multivibrator 181. The output of the monostablemultivibrator 181 activates the line number 1 of the seven-bit shiftregister 177. Since the printing of the characters takes place incolumns 2 through 6 of the seven-column matrix provided for eachcharacter, the output number 1 of the shift register 177 is notconnected to the character decoders 175 but may be used for housekeepingfunctions. When the next line on the mask 159 passes between the lamp151 and the photodiode 167, the pulse output of the photo transistor 167is coupled to the seven-bit shift register 177 through theaforementioned circuitry and causes the shift register 177 to shift,thereby activating output line number 2. This line corresponds to theleftmost column position of the character to be printed and is coupledto column input number 1 of each of the character decoders 175.

The character decoders, on the basis of the six-bit character codesignal present at their inputs from the buffer 173 and the column selectinput present at one of the input lines from the seven-bit shiftregister 177, activate selected ones of their outputs to thecorresponding groups of seven high voltage amplifiers 183, which, inturn, activate the corresponding styli on the corresponding print heads19. In this manner charge dots are deposited on the web 11 for formingthe latent image of the first column of the first character to beprinted. The passage of the successive third through sixth lines on themask 159 between the light source 151 and the phototransistor 167 causethe seven-bit shift register 177 to activate successively its thirdthrough sixth lines, which in turn cause the character 7 decoders 175 toenergize the proper styli on the corresponding print heads 19 throughthe corresponding group of high voltage amplifiers 183 for printing thelatent image of the characters present on the output of the one-linebuffer 173.

When the seventh line on the mask 159 passes between the lamp 151 andthe phototransistor 167, the shift register 177 energizes line number 7which is connected through AND gate 215 (FIG. 15) to OR gate 205 to actas a shift pulse for the shift registers 191 of all four groups 189. TheAND gate 215 is enabled to pass the input from line 7 of the shiftregister 177 since its other input is activated by the multivibrator 213because the slider 19 is moving from left to right.

The shift pulse on line 207 causes the shift registers to shift theircontents end around one place to the right thereby shifting the bits ofthe second character to be printed by each of the print heads into the32nd bit place. This makes the bits of the second character to beprinted present on the inputs of the character decoders 175. Thisprocess continues as the successive lines on the mask 159 pass betweenthe lamp 151 and the phototransistor 167 causing the shift register 177to shift on each line and the shift register 191 of the buffer 173 toshift on every seventh line. In this way each of the print heads arecontrolled through the high voltage amplifiers 183 to latently image theweb 11 with a 32-bit character portion of the l28-character line beingprinted.

When the print heads 17 have printed their 32nd charac-ter anend-of-line pulse is generated. This may be done, for instance, in themanner illustrated in FIG. 17 of the drawings by using a pair ofmagnetically operated reed switches 217 and 219 which are actuated bythe magnetic pole piece 221 mounted on the slider 19. The reed switches217 and 219 are positioned so that when the slider 19 reaches theleft-hand margin of the printed area, switch 217 is closed by the actionof the magnet 221 and when the slider 19 reaches the righthand margin(i.e., when each of the print heads 17 have printed their 32-charactersegment of the line to be printed), the switch 219 is closed by themagnet 221. One side of the reed switches 217 is connected to a voltagesource V, while the other sides are connected to opposite sides of themultivibrator 213 (FIG. Therefore, when the slider 19 reaches thelefthand margin of the area to be printed, indicating that thesubsequent line is to be printed from left to right, the switch 217 setsthe multivibrator 213 so that the output of the multivibrator 213connected to the AND gate 215 is actuated. When the slider reaches therighthand margin, it actuates switch 219 which resets multivibrator 213and actuates its output which is connected to the AND gate 223.

The output pulses from the end-of-line signalling switches 217 and 219are also connected through OR gate 225 to the control logic 195 as anend-of-line signal. This end-of-line signal is sent to the computer onrequest line 227 as a request for the computer to send the next line ofdata to be printed. The computer then sends the next line in the samemanner as was described previously, thereby loading the buffer 173 withthe next line of characters to be printed. After printing a line fromleft to right the next line printed is printed from right to left. Thusthe 32nd, 64th, 96th and 128th characters are printed first by therespective print heads 19. Also each of the printed characters isprinted backwards.

Since the output of the shift registers 191 in the illustratedembodiment is taken from the 32nd bit place which, at the beginning ofthe sweep, holds the character to be printed last by the print headswhen they are printing from right to left, it is necessary to shift thecharacters in the shift registers 191 to supply them to the characterdecoders 175 in the correct order. It is also necessary that the inputsto the character decoders 175 from the seven-bit shift register 177 beactivated in the reverse order from the way in which they are activatedin printing from left to right.

In order to cause the shift register 177 to activate its output inreverse order, it is possible to include gating in the registercontrolled by the output of the multivibrator 213 to reverse the orderof counting, or to include two shift registers, one of which is usedwhen printing from left to right and the other which is used whenprinting from right to left. Either of these alternatives would be wellwithin the skill of one skilled in the art, and therefore will not bedescribed in greater detail here.

In order to bring the bits of the correct character to be printed to the32nd bit place of the registers 191, it is possible to shift thecontents of the register 31 places to the right each time a newcharacter is to be printed. As illustrated in FIG. 15, this may beaccomplished by using a clock circuit 227 which is turned on by output 7of shift register 177 through the AND gate 223 to generate 31 shiftpulses to the OR gate 205. In this way the 31 shift pulses are passed toline 207 and shift the contents of the shift registers 191 by therequired amount. The AND gate 223 is enabled to pass the pulse on itsinput from line 7 of shift register 177 since its input frommultivibrator 215 is actuated. The clock circuit 227 should have a highenough frequency, (e.g. l mhz) in order that the shift registers 191 maybe the requisite 31 places between each character to be printed.

Counter 229 and AND gate 231 are provided to turn off the clock 227 eachtime after it has generated the required 31 shift pulses to the shiftregisters 191. The counter 229 may, for instance, be a standardfive-stage binary counter which counts the pulses from the clock circuit227. The outputs of all five stages are connected to the AND gate 231.When all five stages are actuated, (i.e., when the counter 229 hascounted 31 pulses), the output of AND gate 231 goes high which turns offthe clock 227 and clears the counter 229.

The operation of the electronics is then essentially identical to thatdescribed above in regard to printing from left to right with theexceptions that the shift register 177 counts in the reverse directionand the contents of the shift registers 191 are shifted 31 placesbetween each character place in order to bring the proper character tobe printed to the 32nd bit place of the shift registers 191.

If the computer wants to leave some lines blank instead of printing, itmerely sends a line step signal to the control logic 195 on line 233which causes the control logic to operate the brake portion of theclutch/brake 83 for stopping the slider 19 at the end of its travel andactuates the stepper motor for line spacing the paper. If several linesare to be left blank, the computer sends the required number of blankspace pulses to the control logic for stepping the paper the requiredamount. The slider 19 is kept at one end of its travel for the entireprocedure.

Other signals which may pass between the computer and the control logic195 includes an end-of-print signal on line 237 from the computer to thecontrol logic 195 which tells the printer that the printing is finished.The control logic 195 may, also send an error signal on line 239 to thecomputer when an error condition occurs such as when a paper jam occursor when the paper is exhausted.

An example of a high-voltage amplifier circuit which may be used todrive a stylus on a print head 19 is illustrated in FIG. 18 of thedrawings. Normally high voltage transistor 241 is turned off so thatvoltage source V charges capacitor 243 through resistor 245 and shifteddiode 247 up to approximately V When the character decoder 175 energizesthe input at the base of the transistor 241 with a positive going pulse,the transistor 241 turns on, thereby causing its collector potential todrop to a voltage level slightly above ground. Since the voltage acrosscapacitor 243 cannot change instantaneously, the voltage at the anode ofdiode 247 is driven negatively to a voltage near minus V therebyshutting off diode 247. This negative voltage pulse also appears at thestylus 251 causing a voltage difference of approximately V between thediode 247. This negative voltage pulse also appears at the stylus 251causing a voltage difference of approximately V between the stylus andthe grounded platen 251 thereby causing a dot of negative charge to bedeposited on the dielectric coating of web 11. A V voltage on the orderof between 500 and 800 volts has been found to be suitable for theamplifier. Resistor 253 is provided in order to protect the transistor241 from high current should there be a puncture in the web 1] thatwould short the stylus 249 to ground.

Although the printer, according to the invention, has been described inrelation to a line printer, it should be apparent that with minormodifications the printer could find many other uses. For instance, theprinter could equally well be used for printing graphics if the paperstepping distance were adjusted so that each step was equal to thecharacter height of a character printed by seven styli. In this casesome changes would have to be made in the electronics also in thedecoding of the input and in order to eliminate the intercharacterspaces.

We claim: 1. An electrographic printer for printing dot matrixtypecharacters on a web of dielectric coated paper comprising:

means for moving said web along a path; a printing assembly including aslidable member and a plurality of print heads mounted in said slidablemember, the spaces between adjacent print heads being equal, each ofsaidprint heads having a plurality of styli, said slidable member beingmounted for movement transverse to the path of the said web with saidstyli in contact with the dielectric coated face thereof; 2

means for transporting said slidable member in an oscillatory fashionacrosssaid web; the amplitude of the oscillations being at least equalto the distance between adjacent print heads;

means for selectively energizing said styli for depositing dots ofelectric charge in image configuration on said dielectric coated face ofsaid web as said slidable member traverses said web in either direction,each of said print heads being'operational to print a portion of a linesimultaneously with the printing of other portions of the line by theother print heads of said plurality of print heads;

means coupled to said slidable member for sensing the position of saidslidable member as it traverses said web in either direction and'forenabling said selective energizing means for depositing dots ofelectrical charge on said dielectric coated face of said web only whensaid slidable member is in predetermined positions in its path acrosssaid web, said means for moving said web operating to advance said webby a predetermined amount after each traverse of said slidable memberacross said web; and

processor means for developing said dots of electrical charge depositedby said styli of said print heads.

2. The printer of claim 1 wherein said position sensing means includes:

a light transmitting mask mounted for movement with said slidablemember;

a plurality of parallel opaque lines on said mask, said lines beingspaced by'the intercolumn distance between the columns of dots of saidmatrix-type characters being printed and being oriented orthogonallywith respect to the path of movement of said slidable member;

means fixedly mounted for projecting light through said mask at a pointin its path from one side thereof;

means fixedly mounted opposite said light projecting means on the otherside of said mask for sensing the amount of light transmitted throughsaid mask at a point in its path; and

means responsive to the sensing of the passage of said opaque lines bysaid sensing means for controlling the actuation of said selectiveenergizing means.

3. The printer of claim 2 wherein said light projecting means include alamp and a fiber optic bundle for transmitting light from said lamp to aline portion of said path, said line portion being parallel to saidopaque lines on said mask.

4. The printer of claim 3 wherein the width of said line portion in thepath of said mask is less than the width of said opaque lines on saidmask.

5. The printer of claim 3 wherein said transmitted 'light sensing meansincludes a photosensitive device and a fiber optic bundle for deliveringlight from a line portion of the path of said mask directly opposite theline portion illuminated by the fiber optic bundle of said lightprojecting means to said photo-sensitive device.

6. The printer of claim 1 wherein said means for transporting saidslidable member includes:

a motor; a disc rotated at a constant angular velocity by said motor;cable means connecting a point proximate the periphery of said disc toboth ends of said slidable member; and resilient means connectedinseries with said cable means for compensating for changes in lengthof,

said cable means during the rotation of said disc.

7. The printer of claim 1 wherein said slidable member transportingmeans include a motor, and a clutch brake mechanism between said motorand said slidable member for selectively stopping said slidable memberat either end of its traverse of said web and wherein said printerfurther includes means for causing said web moving means to advance saidweb by a selected amount while said slidable member is held at anextreme of its path by said clutch brake mechanism.

8. The printer of claim 1 wherein said styli on said print heads areoriented in a line perpendicular to the direction of motion of saidprint heads and are spaced from one another by the row distance of thedots of the matrix-type characters to be printed, and further includingan underlining stylus spaced from said plurality of styli andselectively energizable for underlining information printed by saidplurality of writing styli.

9. The printer of claim 1 further including a grounding plate directlyopposite said print heads on the opposite side of said web, saidgrounding plate being urged toward said print heads.

10. The printer of claim 9 wherein said print heads are slidably mountedin said slidable member for movement perpendicular to said web and saidprinting assembly further includes:

means for resiliently biasing said print heads against said web. 1 1.The apparatus of claim 1 wherein said styli of said print heads arearranged in a spaced linear relationship and wherein said print headsfurther include:

insulating layers on either side of said line of spaced styli; and

bearing members having a substantially higher resistance to wear thansaid insulating layers attached to either side of said insulatinglayers, said bearing members, insulating layers and styli being arrangedwith a smooth flush face oriented towards said web.

12. An electrographic printer for printing characters on the web ofdielectric coated paper comprising:

means for moving said web along a path;

a printing assembly including a slider having a printing face and aplurality of groups of linearly arranged spaced styli therein, the endsof said styli being substantially flush with said printing face, saidslider being mounted for oscillatory movement transverse to the path ofsaid web with said printing face in contact with the dielectric coatedface thereof;

means for transporting said slider in an oscillatory fashion across saidweb;

means for selectively energizing said styli for depositing dots of anelectric charge in image configuration on the said dielectric coatedface of said web as said slider traverses said web in both directions,said groups of said styli being arranged in equal spaced relationshipwith one another for depositing charge on equal adjacent segments ofsaid web, each of said groups of styli being operational to print aportion of a line simultaneously with the printing of the other portionsof the line by the other groups of styli; and

processor means for developing said dots of electric charge deposited bysaid styli.

* III

1. An electrographic printer for printing dot matrix-type characters ona web of dielectric coated paper comprising: means for moving said webalong a path; a printing assembly including a slidable member and aplurality of print heads mounted in said slidable member, the spacesbetween adjacent print heads being equal, each of said print headshaving a plurality of styli, said slidable member being mounted formovement transverse to the path of the said web with said styli incontact with the dielectric cOated face thereof; means for transportingsaid slidable member in an oscillatory fashion across said web; theamplitude of the oscillations being at least equal to the distancebetween adjacent print heads; means for selectively energizing saidstyli for depositing dots of electric charge in image configuration onsaid dielectric coated face of said web as said slidable membertraverses said web in either direction, each of said print heads beingoperational to print a portion of a line simultaneously with theprinting of other portions of the line by the other print heads of saidplurality of print heads; means coupled to said slidable member forsensing the position of said slidable member as it traverses said web ineither direction and for enabling said selective energizing means fordepositing dots of electrical charge on said dielectric coated face ofsaid web only when said slidable member is in predetermined positions inits path across said web, said means for moving said web operating toadvance said web by a predetermined amount after each traverse of saidslidable member across said web; and processor means for developing saiddots of electrical charge deposited by said styli of said print heads.2. The printer of claim 1 wherein said position sensing means includes:a light transmitting mask mounted for movement with said slidablemember; a plurality of parallel opaque lines on said mask, said linesbeing spaced by the intercolumn distance between the columns of dots ofsaid matrix-type characters being printed and being orientedorthogonally with respect to the path of movement of said slidablemember; means fixedly mounted for projecting light through said mask ata point in its path from one side thereof; means fixedly mountedopposite said light projecting means on the other side of said mask forsensing the amount of light transmitted through said mask at a point inits path; and means responsive to the sensing of the passage of saidopaque lines by said sensing means for controlling the actuation of saidselective energizing means.
 3. The printer of claim 2 wherein said lightprojecting means include a lamp and a fiber optic bundle fortransmitting light from said lamp to a line portion of said path, saidline portion being parallel to said opaque lines on said mask.
 4. Theprinter of claim 3 wherein the width of said line portion in the path ofsaid mask is less than the width of said opaque lines on said mask. 5.The printer of claim 3 wherein said transmitted light sensing meansincludes a photosensitive device and a fiber optic bundle for deliveringlight from a line portion of the path of said mask directly opposite theline portion illuminated by the fiber optic bundle of said lightprojecting means to said photo-sensitive device.
 6. The printer of claim1 wherein said means for transporting said slidable member includes: amotor; a disc rotated at a constant angular velocity by said motor;cable means connecting a point proximate the periphery of said disc toboth ends of said slidable member; and resilient means connected inseries with said cable means for compensating for changes in length ofsaid cable means during the rotation of said disc.
 7. The printer ofclaim 1 wherein said slidable member transporting means include a motor,and a clutch brake mechanism between said motor and said slidable memberfor selectively stopping said slidable member at either end of itstraverse of said web and wherein said printer further includes means forcausing said web moving means to advance said web by a selected amountwhile said slidable member is held at an extreme of its path by saidclutch brake mechanism.
 8. The printer of claim 1 wherein said styli onsaid print heads are oriented in a line perpendicular to the directionof motion of said print heads and are spaced from one another by the rowdistance of the dots of the matrix-type characters to be printed, andfurther including an underlining stylus spaced from said plurality ofstyli and selectively energizable for underlining information printed bysaid plurality of writing styli.
 9. The printer of claim 1 furtherincluding a grounding plate directly opposite said print heads on theopposite side of said web, said grounding plate being urged toward saidprint heads.
 10. The printer of claim 9 wherein said print heads areslidably mounted in said slidable member for movement perpendicular tosaid web and said printing assembly further includes: means forresiliently biasing said print heads against said web.
 11. The apparatusof claim 1 wherein said styli of said print heads are arranged in aspaced linear relationship and wherein said print heads further include:insulating layers on either side of said line of spaced styli; andbearing members having a substantially higher resistance to wear thansaid insulating layers attached to either side of said insulatinglayers, said bearing members, insulating layers and styli being arrangedwith a smooth flush face oriented towards said web.
 12. Anelectrographic printer for printing characters on the web of dielectriccoated paper comprising: means for moving said web along a path; aprinting assembly including a slider having a printing face and aplurality of groups of linearly arranged spaced styli therein, the endsof said styli being substantially flush with said printing face, saidslider being mounted for oscillatory movement transverse to the path ofsaid web with said printing face in contact with the dielectric coatedface thereof; means for transporting said slider in an oscillatoryfashion across said web; means for selectively energizing said styli fordepositing dots of an electric charge in image configuration on the saiddielectric coated face of said web as said slider traverses said web inboth directions, said groups of said styli being arranged in equalspaced relationship with one another for depositing charge on equaladjacent segments of said web, each of said groups of styli beingoperational to print a portion of a line simultaneously with theprinting of the other portions of the line by the other groups of styli;and processor means for developing said dots of electric chargedeposited by said styli.